AQUIFER CHARACTERIZATION REPORT TASK 5 OF AQUIFER … · 2007. 12. 28. · • Task 4 – Sulfate Fate and Transport Evaluation • Task 5 – Preparation of the Aquifer Characterization

AQUIFER CHARACTERIZATION REPORT

TASK 5 OF AQUIFER CHARACTERIZATION PLAN MITIGATION ORDER ON CONSENT DOCKET NO. P-50-06

PIMA COUNTY, ARIZONA

Prepared for:

PHELPS DODGE SIERRITA, INC. 6200 West Duval Mine Road Green Valley, Arizona 85614

Prepared by:

HYDRO GEO CHEM, INC. 51 West Wetmore Road Tucson, Arizona 85705

(520) 293-1500

December 28, 2007

Aquifer Characterization Report G:\783000\REPORTS\ACR\Aquifer Characterization Rpt.doc December 28, 2007

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TABLE OF CONTENTS

1. INTRODUCTION .............................................................................................................. 1 1.1 Purpose of the Aquifer Characterization Report..................................................... 1 1.2 Scope of the Aquifer Characterization Report........................................................ 2 1.3 Organization of Report ........................................................................................... 4

2. RESULTS OF AQUIFER CHARACTERIZATION PLAN TASKS 1, 2, AND 3 ........... 7

2.1 Task 1 - Well Inventory.......................................................................................... 7 2.2 Task 2 - Plume Characterization............................................................................. 9

2.2.1 Task 2.1 - Data Compilation and Evaluation.............................................. 9 2.2.2 Task 2.2 - Groundwater Monitoring ......................................................... 10

2.2.2.1 Overview of Groundwater Monitoring Program....................... 10 2.2.2.2 Sulfate Distribution ................................................................... 12 2.2.2.3 Groundwater Elevation ............................................................. 13

2.2.3 Task 2.3 - Depth-Specific Sampling......................................................... 14 2.2.4 Task 2.4 - Offsite Well Installation and Testing....................................... 16

2.2.4.1 Well Drilling and Installation.................................................... 16 2.2.4.2 Hydraulic Testing of MO-2007 Monitor Wells ........................ 20 2.2.4.3 Initial Sampling of MO-2007 Monitor Wells ........................... 21

2.3 Task 3 - Evaluation of PDSI Groundwater Control System................................. 23 3. CONCEPTUAL MODEL FOR THE GROUNDWATER SULFATE PLUME.............. 27

3.1 Sulfate Sources ..................................................................................................... 27 3.2 Sulfate Migration .................................................................................................. 30

3.2.1 Hydrostratigraphy of the Basin Fill Aquifer............................................. 30 3.2.2 Sulfate Distribution in the Basin Fill Aquifer........................................... 35 3.2.3 Sulfate Transport....................................................................................... 35

4. NUMERICAL MODEL OF GROUNDWATER FLOW AND TRANSPORT............... 37 5. CONCLUSIONS .............................................................................................................. 41 6. REFERENCES ................................................................................................................. 43 7. LIMITATIONS STATEMENT........................................................................................ 45

TABLES 1 Summary of MO-2007-Series Wells 2 Summary of Hydraulic Parameters for MO-2007-Series Wells 3 Water Quality Data for Initial Sampling of MO-2007-Series Wells


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TABLE OF CONTENTS (Continued)

FIGURES 1 Location of the Sulfate Plume Based on Information as of October 2007 2 Drinking Water Supply Wells Identified and Sampled for the Well Inventory 3 Contour Map of Kriged Bedrock Elevations Based on Borehole Data 4 Sulfate Concentrations in Groundwater Samples Collected in July through October 2007 5 Groundwater Elevations for July through October 2007 6 Simulated Groundwater Level Contours for the End of 2006 with Measured Groundwater

Levels from July through October 2007 7 Simulated Sulfate Concentration Contours with Measured Sulfate Concentrations from

Third Quarter 2007

APPENDICES

A Data Compilation and Evaluation of Bedrock Elevations and Hydraulic Tests for Numerical Model Development in the Vicinity of the Phelps Dodge Sierrita Tailing Impoundment, Task 2.1 of Aquifer Characterization Plan

B Summary of Water Quality and Water Level Data Collected for Task 2.2 of Aquifer Characterization Plan

C Depth-Specific Water Sampling and Inflow Profiling at Existing Wells in the Vicinity of the Phelps Dodge Sierrita Tailing Impoundment, Task 2.3 of Aquifer Characterization Plan

D Results of Monitoring Well Installation, Task 2.4 of Aquifer Characterization Plan E Evaluation of Hydraulic Tests at MO-2007-Series Wells, Task 2.4 of Aquifer

Characterization Plan F Results of Initial Water Quality Sampling at Offsite Monitoring Wells, Task 2.4 of

Aquifer Characterization Plan G Geologic Cross Sections H Cross Sections Showing Water Quality and Hydraulic Conductivity Data I Numerical Model for Simulation of Groundwater Flow and Sulfate Transport in the

Vicinity of the Phelps Dodge Sierrita Tailing Impoundment, Task 4 of Aquifer Characterization Plan


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1. INTRODUCTION

In June 2006, Phelps Dodge Sierrita, Inc. (PDSI) and Arizona Department of

Environmental Quality (ADEQ) entered into Mitigation Order on Consent Docket No. P-50-06.

The Mitigation Order requires PDSI to characterize the extent of a groundwater sulfate plume

(defined as sulfate concentrations in excess of 250 milligrams per liter (mg/L)) originating from

the Phelps Dodge Sierrita Tailing Impoundment (PDSTI) (Figure 1) and to develop a Mitigation

Plan for impacted drinking water supplies attributable to the PDSTI.

Pursuant to the Mitigation Order, PDSI submitted to ADEQ the Work Plan to

Characterize and Mitigate Sulfate in Drinking Water Supplies in the Vicinity of the Phelps

Dodge Sierrita Tailing Impoundment (Work Plan) (Hydro Geo Chem, Inc. (HGC), 2006a).

ADEQ approved the Work Plan in a letter dated November 15, 2006 (ADEQ, 2006), initiating its

implementation by PDSI. The Aquifer Characterization Plan is a component of the Work Plan

that specifies work to better characterize the hydrogeology and water quality of the sulfate

plume. The Work Plan also provides for a Feasibility Study to evaluate potential mitigation

actions for a Mitigation Plan.

1.1 Purpose of the Aquifer Characterization Report

The Aquifer Characterization Report is a requirement of Section III.C of the Mitigation

Order and presents the results of hydrologic investigations conducted from November 2006

through December 2007 as prescribed by the Aquifer Characterization Plan contained in the


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Work Plan. As described in the Work Plan, the results of the Aquifer Characterization Plan

provide information needed to complete the Feasibility Study and Mitigation Plan for

sulfate-impacted drinking water supplies. HGC prepared the Work Plan, conducted Aquifer

Characterization Plan investigations identified in the Work Plan, and prepared this report under

contract to PDSI.

1.2 Scope of the Aquifer Characterization Report

As stated in the Work Plan, the objectives of the Aquifer Characterization Plan are to

address the Mitigation Order requirements to characterize the sulfate plume and to collect data to

complete the Feasibility Study. Specifically, the objectives included the following requirements

of Sections III.A and III.C of the Mitigation Order:

$ Complete a well inventory to identify drinking water wells within one mile downgradient and cross-gradient of the outer edge of the sulfate plume.

$ Determine the vertical and horizontal extent of the sulfate plume.

$ Evaluate the fate and transport of the outer edge of the sulfate plume.

$ Evaluate the effectiveness of the interceptor wellfield as a groundwater sulfate control

system.

Based on an analysis of Mitigation Order requirements and data needs, the Aquifer

Characterization Plan includes five tasks as follows:

• Task 1 – Well Inventory

• Task 2 – Plume Characterization


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o Task 2.1 Data Compilation and Evaluation o Task 2.2 Groundwater Monitoring o Task 2.3 Depth-Specific Groundwater Sampling at Existing Wells o Task 2.4 Offsite Well Installation and Testing

• Task 3 – Evaluation of PDSI’s Sulfate Control System

• Task 4 – Sulfate Fate and Transport Evaluation

• Task 5 – Preparation of the Aquifer Characterization Report

The Work Plan detailed the scope, methods, and reporting schedule for these tasks, which

include field and office activities conducted by HGC and others. Reports for Tasks 1, 2.2, and 3

have been previously reported to ADEQ in the following submittals:

• Well Inventory Report for Task 1 of Aquifer Characterization Plan for Mitigation

Order on Consent No. P-50-06 dated December 20, 2006 by HGC (HGC, 2006b).

• Groundwater Monitoring Report, Fourth Quarter 2006, Tasks 2.2 and 2.3 of Aquifer Characterization Plan, Mitigation Order on Consent No. P-50-06 dated December 29, 2006 by HGC (HGC, 2006d).

• Evaluation of the Current Effectiveness of the Sierrita Interceptor Wellfield, Phelps

Dodge Sierrita Mine, Pima County, Arizona dated February 26, 2007 by Errol L. Montgomery & Associates, Inc. (M&A) (M&A, 2007a).

• First Quarter 2007 Groundwater Monitoring Report, Tasks 2.2 and 2.3 of Aquifer

Characterization Plan, Mitigation Order on Consent No. P-50-06 dated March 30, 2007 by HGC (HGC, 2007a).

• Second Quarter 2007 Groundwater Monitoring Report, Tasks 2.2 and 2.3 of Aquifer

Characterization Plan, Mitigation Order on Consent No. P-50-06 dated June 28, 2007 by HGC (HGC, 2007b).

• Third Quarter 2007 Groundwater Monitoring Report, Tasks 2.2, 2.3, and 2.4 of

Aquifer Characterization Plan, Mitigation Order on Consent No. P-50-06 dated September 26, 2007 by HGC (HGC, 2007c).

• Revised Report: Evaluation of the Current Effectiveness of the Sierrita Interceptor

Wellfield, Phelps Dodge Sierrita Mine, Pima County, Arizona dated November 14, 2007 by M&A (M&A, 2007b).


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For completeness, this report summarizes the results of previously reported work

conducted under the Aquifer Characterization Plan, but will not reproduce previously submitted

reports. Previously submitted reports are available at the information repository at the

Joyner-Green Valley Branch Library or from the PDSI document library website

(http://www.phelpsdodge.com/caglibrary). This report does provide complete task reports for

the previously unreported Tasks 2.1, 2.3, 2.4, and 4. The Aquifer Characterization Report itself

is the deliverable for Task 5.

Background information on the Mitigation Order, the nature of the sulfate plume, the

hydrogeology and water quality of the sulfate plume, and mitigation activities at the interceptor

wellfield are available from the Work Plan and will not be repeated here except as needed to

report work results or to describe the conceptual model for the sulfate plume. The Work Plan

contained a preliminary conceptual model of the sulfate plume which was updated based on

information from investigations conducted pursuant to the Aquifer Characterization Plan.

1.3 Organization of Report

Section 2 summarizes the results of Tasks 1, 2, and 3, namely, the well inventory, plume

characterization, and evaluation of the interceptor wellfield. Section 3 discusses the revised

conceptual model based on these results. Section 4 presents the results of numerical modeling of

the sulfate plume conducted for Task 4. Section 5 summarizes the accomplishments of work

conducted under the Aquifer Characterization Plan.


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The Appendices contain individual task reports for Tasks 2.1, 2.3, 2.4, and 4 as follows:

• Appendix A - Data Compilation and Evaluation of Bedrock Elevations and Hydraulic

Tests for Numerical Model Development in the Vicinity of the Phelps Dodge Sierrita Tailing Impoundment, Task 2.1 of Aquifer Characterization Plan

• Appendix B - Summary of Water Quality and Water Level Data Collected for Task

2.2 of Aquifer Characterization Plan

• Appendix C - Depth-Specific Water Sampling and Inflow Profiling at Existing Wells in the Vicinity of the Phelps Dodge Sierrita Tailing Impoundment, Task 2.3 of Aquifer Characterization Plan

• Appendix D - Results of Monitoring Well Installation, Task 2.4 of Aquifer

Characterization Plan

• Appendix E - Evaluation of Hydraulic Tests at MO-2007-Series Wells, Task 2.4 of Aquifer Characterization Plan

• Appendix F - Results of Initial Water Quality Sampling at Off-Site Monitoring Wells

Installed for Task 2.4 of Aquifer Characterization Plan

• Appendix G - Geologic Cross Sections

• Appendix H - Cross Sections Showing Water Quality and Hydraulic Conductivity Data

• Appendix I - Numerical Model for Simulation of Groundwater Flow and Sulfate

Transport in the Vicinity of the Phelps Dodge Sierrita Tailing Impoundment, Task 4 of Aquifer Characterization Plan


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2. RESULTS OF AQUIFER CHARACTERIZATION PLAN TASKS 1, 2, AND 3

Aquifer Characterization Plan Tasks 1, 2, and 3 include the well inventory, plume

characterization activities, and the evaluation of the effectiveness of the interceptor wellfield

operated by PDSI to mitigate the sulfate plume.

2.1 Task 1 - Well Inventory

The objective of the well inventory was to identify and sample drinking water supply

wells within one mile of the downgradient and crossgradient edge of the sulfate plume from the

PDSTI (Figure 2). The well inventory also evaluated the presence of drinking water wells within

the footprint of the plume. The results of the well inventory were reported by HGC (2006b).

The well inventory identified 165 wells within one mile of the sulfate plume of which

10 were active drinking water supply wells (Figure 2). The drinking water supply wells were

identified using the following steps:

• Compilation and review of data for wells registered with the Arizona Department of

Water Resources (ADWR).

• Cross checking of the registered wells with information from databases for ADWR water providers and ADEQ public water systems.

• Compilation and review of ADWR imaged records for potential drinking water

supply wells.

• Field checking of potential drinking water supply wells.

• Contacting the owners/operators of potential drinking water supply wells.


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Of the 10 active drinking water supply wells, one was a private domestic supply well and

nine were public supply wells. A water quality sample was collected from the private domestic

well which was subsequently determined to be outside the inventory area based on its location

determined by a global positioning system. Water quality data for the nine wells serving as

public supply wells were provided by the owners or operators. Samples from the 10 drinking

water supply wells had sulfate concentrations less than the limit of 250 mg/L set by the

Mitigation Order except for well ESP-1, which had a sulfate concentration of 262 mg/L in a

sample collected on December 4, 2006. At the time of the well inventory, water from ESP-1 was

blended with water from ESP-2 and ESP-3 in a storage tank to reduce the concentration of the

blended water to less than 250 mg/L prior to distribution for use. Wells ESP-1, ESP-2, and

ESP-3 were used as drinking water supply only temporarily while additional wells were being

developed by Community Water Company (CWC). Use of wells ESP-1, ESP-2, and ESP-3 to

provide drinking water was discontinued in 2007 after CWC placed wells CW-10 and CW-11

into service. Ongoing sampling of public drinking water supply wells was conducted quarterly

under Task 2.2 (Section 2.2.2).

In conjunction with the well inventory and pursuant to the Work Plan, a technical

memorandum (HGC, 2006c) was submitted to ADEQ describing the potential interim actions

PDSI would take if a drinking water supply was found to be impacted due to the PDSTI. The

memorandum describes a monitoring program implemented for sulfate in drinking water

supplies, sulfate levels that would trigger an interim action, and the process to be followed to

select and implement any needed potential interim action.


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2.2 Task 2 - Plume Characterization

Plume characterization activities for Task 2 consisted of data compilation and evaluation

activities as well as field investigations. The data compilation and evaluation activities focused

on assembling and evaluating existing data that would be used to characterize the structure and

hydraulic properties of the basin fill aquifer containing the plume. The field investigations

focused on characterizing current water level and water quality conditions in the regional aquifer,

installing monitoring wells to determine the vertical and lateral distribution of the sulfate plume,

and testing monitoring wells to estimate aquifer hydraulic properties.

2.2.1 Task 2.1 - Data Compilation and Evaluation

An evaluation of available bedrock elevation data and hydraulic test results was

performed under Task 2.1. The purpose of the evaluation was to compile, evaluate, and verify

data on the depth and hydraulic properties of the basin fill aquifer. These data are needed to

develop and calibrate a numerical groundwater flow model for the site. The results of the data

compilation and evaluation are detailed in Appendix A.

The purpose of the bedrock evaluation was to develop a bedrock elevation database for

the southern portion of the Tucson basin using well and borehole data and to construct a bedrock

elevation contour map. Figure 3 is a contour map of subsurface bedrock elevations based on a

geostatistical interpretation of bedrock depth data from wells and boreholes.


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Hydraulic test data were compiled and evaluated to verify the hydraulic properties of the

basin fill and bedrock in the vicinity of the PDSTI. As reported in Appendix A, pumping test

and slug test data were obtained from the reports of various hydrologic studies conducted in the

PDSTI and Green Valley area. HGC analyzed data from pumping tests and slug tests to verify

aquifer hydraulic properties including transmissivity and hydraulic conductivity data reported in

the Work Plan. Appendix A contains summary tables with the results of the hydraulic test

analyses. The hydraulic properties data reported in the Work Plan were determined to be

suitable for use in aquifer characterization.

2.2.2 Task 2.2 - Groundwater Monitoring

Groundwater monitoring for Task 2.2 consisted of groundwater sample collection and

water elevation measurement from wells in the vicinity of the PDSTI. Data for the fourth quarter

of 2006 (HGC, 2006d) and the first (HGC, 2007a), second (HGC, 2007b), and third

(HGC, 2007c) quarters of 2007 were collected and reported under the groundwater monitoring

program. Appendix B contains tables summarizing water quality and water level measurements

for Task 2.2 from fourth quarter 2006 through third quarter 2007.

2.2.2.1 Overview of Groundwater Monitoring Program

The Work Plan identified two purposes for the groundwater monitoring program for

Task 2.2, namely, plume monitoring and regional monitoring. Plume monitoring was conducted

quarterly at wells near the boundary of the sulfate plume to track its location. Regional


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monitoring was conducted twice, in the first and third quarters of 2007, to characterize regional

hydrologic and water quality conditions during high (summer) and low (winter) seasonal

pumping periods.

PDSI and HGC conducted the majority of the groundwater monitoring pursuant to

Task 2.2. Groundwater sampling and analysis methods used by PDSI and HGC are described in

the Quality Assurance Project Plan (Appendix E of HGC, 2006a). Some groundwater

monitoring data were reported to PDSI by other parties that may have used different, but

comparable, sampling protocols. Data verification reports were prepared for each quarterly

report for quality assurance and quality control purposes. As determined by the analytical data

verification review, all groundwater monitoring data collected for Task 2.2 are of acceptable

quality for use in the aquifer characterization program.

Plume monitoring for Task 2.2 is ongoing. Monitoring wells installed under Task 2.4

(Section 2.2.4) were added to the plume monitoring program as soon as they were completed.

Water quality sampling at the new wells has helped to define the eastern extent of the sulfate

plume and the vertical distribution of sulfate. The results of the two regional monitoring events

in the first and third quarters of 2007 provided water level and water quality data sets with broad

geographic coverage. For example, in the third quarter of 2007, water level measurements were

collected at 134 wells and water quality samples were collected from 108 wells covering an area

of 50 square miles.


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The results of individual monitoring events are presented and discussed in the quarterly

groundwater monitoring reports (HGC, 2006d, 2007a, 2007b, and 2007c). The results of

regional monitoring in the third quarter 2007 are used to illustrate sulfate concentration and

water elevation trends because they are the most complete and exhibit the same general trends

observed in the previous monitoring events. Figures 4 and 5 show sulfate concentration and

groundwater elevation data for the third quarter of 2007 (HGC, 2007c) augmented with data for

newly installed wells (Section 2.2.4). Sulfate concentration and water elevation maps for the

first quarter of 2007 (HGC, 2007a and 2007b) are contained in Appendix B for comparison.

2.2.2.2 Sulfate Distribution

Figure 4 shows the regional distribution of sulfate concentrations in samples collected

from wells in the basin fill aquifer. The concentration contours shown in Figure 4 are inferred

assuming that sulfate concentrations in the aquifer are spatially related, although a strict linear

interpolation was not applied. The contours on Figures 1 and 4 were developed using the highest

measured sulfate concentrations at co-located wells.

The sulfate concentration data for third quarter 2007 provide the most complete

description of the sulfate plume associated with the PDSTI available to date. Groundwater

sample results indicate that the northern extent of the plume is north of Duval Mine Road and

west of La Canada Drive, as indicated on Figure 4, which is consistent with the extent of the

plume shown in previous reports (HGC, 2006a, 2006d, 2007a, and 2007b). The initial sulfate

analyses from reconditioned wells TMM-1, NP-2, and CW-3, and newly installed wells


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MO-2007-1A, -1B, -1C, -2, -3B, -3C, -4A, -4B, -4C, -5B, -5C, -6A, and -6B provide a better

definition of the northern and eastern edges of the plume than previously available. The initial

sulfate concentrations detected in samples collected from the newly installed wells will be

confirmed by subsequent monitoring.

Groundwater samples with sulfate concentrations less than 50 mg/L sulfate define a

north-south zone approximately 6 miles long and ranging from 1,400 to 6,000 feet wide east of

the sulfate plume. This zone of low sulfate groundwater is centered on Green Valley and

extends north of Duval Mine Road along Interstate 19. Sulfate concentrations less than 10 mg/L

are contained in groundwater samples from wells south of the PDSTI and west of Interstate 19.

Samples from wells along the channel of the Santa Cruz River east of Interstate 19 had sulfate

concentrations ranging between approximately 60 mg/L and 160 mg/L. Sulfate concentrations

are generally less than 100 mg/L in samples collected from wells on the alluvial fan from the

Santa Rita Mountains east of the Santa Cruz River channel. Groundwater samples collected

from wells farthest east on the alluvial fan of the Santa Rita Mountains had sulfate

concentrations less than 50 mg/L.

2.2.2.3 Groundwater Elevation

Groundwater elevations are shown on Figure 5. Appendix B contains maps of depth to

water and basin fill saturated thickness for the third quarter of 2007 (Figures B.3 and B.4,

respectively). Groundwater elevations decrease eastward from the immediate vicinity of PDSTI,

from south to north across the central portion of the study area near Green Valley, and from east


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to west on the alluvial fan east of the Santa Cruz River. Based on the groundwater elevation

contours, groundwater flows from the flanks of the Santa Rita on the east and Sierrita Mountains

on the west toward the central axis of the basin, and then northerly. This overall pattern of

groundwater flow is consistent with expected regional groundwater flow patterns in the southern

portion of the Tucson groundwater basin. The groundwater elevations and consequent flow

directions indicated in the vicinity of the PDSTI are generally consistent with data for 2005/2006

(HGC, 2006a) and 1993/1994 (M&A and Dames & Moore, 1994).

Comparison of the third quarter 2007 water elevations with those shown in the Work Plan

for 2005/2006 and with those in the groundwater monitoring reports for the fourth quarter 2006

(HGC, 2006d), first quarter 2007 (HGC, 2007a), and second quarter 2007 (HGC, 2007b)

indicates no substantive difference in groundwater elevations and consequent flow directions

over this time range.

2.2.3 Task 2.3 - Depth-Specific Sampling

Depth-specific sampling and flow velocity profiling at existing wells in the vicinity of

PDSTI were conducted between November 9, 2006 and June 7, 2007. The purpose of the depth-

specific sampling and flow velocity profiling was to delineate aquifer characteristics, including

water quality variations and changes in relative permeability with depth.

Depth-specific sampling was conducted at long-screened monitoring wells MH-11 and

MH-12 in accordance with the Work Plan. Depth-specific sampling and flow velocity profiling


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were completed at wells ESP-2 and ESP-4. Attempts to conduct sampling and profiling at wells

CW-7, CW-8, ESP-1, and ESP-3 were unsuccessful due to the inability of the sampling tool to

access the entire depth of the wells as a result of the configuration of existing equipment in the

pumps and riser piping in the wells.

The results and interpretation of sampling for Task 2.3 are presented in Appendix C. The

salient findings of sampling for Task 2.3 are:

$ A zone of relatively high permeability at ESP-2 appears to be present at depths between

650 and 700 feet.

$ A zone of relatively high permeability in the vicinity of ESP-4 appears to be present at depths from about 650 feet to about 800 feet.

$ ESP-4 lies within the zone of sulfate impact, and the most elevated sulfate concentrations

are centered between depths of 750 and 800 feet, corresponding to the zone of higher apparent permeability.

$ Sulfate concentrations in samples from MH-11 (screened from 300 to 800 feet below

ground surface (ft bgs) and sampled from 450 to 750 ft bgs) and MH-12 (screened from 280 to 800 ft bgs and sampled from 470 to 700 ft bgs) are consistent from top to bottom of the intervals samples.

The general implications of these findings is that there can be zones of higher relative

permeability at depth in the basin fill aquifer that impart heterogeneity to the basin fill. A strong

vertical zoning of sulfate was evident at ESP-4 where the high permeability zone was associated

with an order of magnitude increase in sulfate concentrations. The uniformity and continuity of

the high permeability zones is uncertain given the large distances between wells.


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2.2.4 Task 2.4 - Offsite Well Installation and Testing

Pursuant to Task 2.4, HGC conducted drilling, construction, and testing of thirteen water

quality monitoring wells in areas east and northeast of the PDSTI in and near the community of

Green Valley, Arizona. Monitor wells were installed to:

• Further define the lateral extent of the sulfate plume. • Define the vertical zoning of sulfate.

• Provide installations for long term monitoring of water levels and water quality.

• Characterize aquifer materials and hydraulic properties in the basin fill aquifer.

• Determine depth to bedrock and thickness of the basin fill at each location.

Monitoring wells were installed at six locations, MO-2007-1 through MO-2007-6,

located east and northeast of the PDSTI (Figures 1, 4, and 5). The sites were selected to provide

additional definition of the plume limits at their respective locations. Table 1 summarizes the

well construction of the MO-2007-series wells.

2.2.4.1 Well Drilling and Installation

Monitor well installation was focused at the northern and eastern portions of the plume

because groundwater flow downgradient from the PDSTI is to the east and then north, and

because these areas had the greatest uncertainty regarding the distribution of sulfate and are of

concern with respect to future plume migration. Some of the well sites were selected so that the

wells can serve as sentinel wells for water supply wells near the current plume margin.


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Appendix D details the drilling, construction, and development of the MO-2007-series

monitoring wells and provides a summary of the geology, the rationale for well screen selection,

drilling logs, well construction diagrams, and well development information.

Nests of two to three wells were installed at all sites except MO-2007-2 to assess vertical

differences in hydraulic properties and sulfate distribution in the basin fill aquifer. Only one well

was installed at MO-2007-2 because the saturated thickness of the basin fill is insufficient to

warrant multiple screened intervals. The well nests allow sampling and hydrologic testing of

specific vertical intervals within the basin fill. Selection of screened intervals for the monitor

well nests was based on two primary criteria. First, the screened intervals were positioned to

monitor the top, middle, and bottom of the basin fill with the shallow (“A”), middle (“B”), and

deep (“C”), respectively, to follow the pattern that had been established for some MH-series

monitor wells. Second, lithological and water quality information provided by pilot boreholes

drilled from the surface to bedrock at each site was used to select specific hydrostratigraphic

zones to include or avoid in the screened intervals in a particular well. Access to pre-existing

shallow wells NP-2 and CW-3 was obtained at sites MO-2007-3 and MO-2007-5, respectively,

eliminating the need to install shallow wells at these locations.

Pilot boreholes drilled at the MO-2007 sites intercepted Quaternary- to Tertiary-aged

basin fill deposits overlying Cretaceous clastic sedimentary and volcanic bedrock. The basin fill

is composed of unconsolidated to moderately consolidated sand, silt, gravel, and clay. Basin fill

thicknesses encountered in the pilot boreholes drilled to bedrock ranged from a minimum of

687 feet in MO-2007-2 to a maximum of 1,442 feet in MO-2007-3C. Depth to bedrock and


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bedrock lithology encountered in the MO-2007 pilot boreholes are summarized in Table D.1

(Appendix D).

Identification of stratigraphy in the basin fill was an objective of Task 2.4 because the

physical characteristics of the basin fill, such as the presence or absence of consistent layering or

laterally extensive zones of fine- or coarse-grained materials can be controls on the hydraulic

properties of basin fill and influence the movement of groundwater and solutes. Appendix D

contains a generalized stratigraphic section for each MO-2007 pilot borehole. The stratigraphic

sections were developed by grouping together the predominant material types and interpreting

transition breaks between the groups. The generalized stratigraphic sections are meant to show

only the general tendencies of the basin fill at the well sites because the grouping of materials

can be somewhat subjective due to the discontinuous and disrupted nature of samples collected,

the generally coarse-grained character of the deposits, and the gradational and sporadic transition

between interpreted groups of materials.

Five of the MO-2007-series well sites are located along the northeastern and eastern

margin of the sulfate plume (e.g., MO-2007-1, -3, -4, -5, and -6) and one site, MO-2007-2, is

located in the northwestern portion of the plume. Geologic data for wells along the northeastern

and eastern margin of the plume were amenable to interpretation of generalized stratigraphic

units in the basin fill.

A consistent aspect of the basin fill observed in the MO-2007-series well sites is that the

uppermost 200 to 450 feet contained a significant fraction of silt and clay. This was not


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observed in MO-2007-2 on the northwest side of the plume. The upper zone of basin fill is

composed of mixed sand, gravel, silt, and clay, but has a greater occurrence of silt or clay layers

intermixed with layers of silty sand or gravel compared to the underlying material. Within the

interpreted upper zone there are lateral variations such as at MO-2007-5 which has less silt and

clay than observed at the other MO-2007 wells on the east side of the plume. The upper zone of

basin fill typically extends from the surface to the vicinity of the water table. The upper portion

of the basin fill at MO-2007-2 was sand that lacked the higher content of fines observed at the

other MO-2007 wells. For this reason, the upper zone of the basin fill may not extend to

MO-2007-2.

Below the upper zone, a middle zone of the basin fill consists of predominantly coarse-

grained sediments containing various sand and gravel mixtures. In general, the middle zone is

characterized by higher percentages of sand and gravel compared to overlying and underlying

materials. Silt is a subsidiary component of the middle zone and increases from north to south

based on the observation that layers of sand with silt or silty sand occur more commonly at sites

MO-2007-4, -5, and -6 than at sites MO-2007-1 and -3.

A lower zone of the basin fill can be inferred on the basis of sediment characteristics and

drilling characteristics, although the lateral consistency of the lower unit is more variable than

the overlying units. One characteristic of the lower zone is a general lack of gravel. At sites

MO-2007-1, -5, and -6, a lower zone of silty sand and sand with silt underlies the coarse-grained

middle zone. The lower portions of MO-2007-3 and 4 were sand that contrasted with overlying

material due to the lack of gravel and the relative uniformity of the sand. Other characteristics of


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the lower unit are 1) the materials in it, whether silty or sandy, were periodically associated with

slow drilling conditions (e.g., a “hard formation” penetration rate of less than 5 feet per hour for

a continuous period of 2 hours); 2) sediment is moderately indurated in places (e.g., sites

MO-2007-4 and -5); and 3) greater calcium carbonate relative to overlying material (e.g., sites

MO-2007-1, -3, and -4) as determined by testing with hydrochloric acid. Although there are

enough apparent features to differentiate the lower zone from the overlying material at each well

site, the lower zone appears to vary laterally in its silt and clay content and degree of induration.

In general, site MO-2007-3 on the northeast margin of the plume contains the thickest

assemblage of sand and sand with gravel, whereas site MO-2007-6, southeast of the plume,

contains the least amount of clean sand and gravel. In Section 3.2.1, the stratigraphy of the

MO-2007-series well sites is discussed in the context of previously collected geologic

information.

2.2.4.2 Hydraulic Testing of MO-2007 Monitor Wells

Aquifer testing was conducted at each of the MO-2007 monitor wells following their

development. The purpose of the tests was to evaluate basin fill aquifer hydraulic properties,

including transmissivity, vertical hydraulic conductivity, and storage coefficient in the vicinity of

each well nest. Appendix E reports the results of the aquifer testing program in detail, including

discussion of the test methods, presentation of drawdown graphs, and interpretation of the results

of tests. Table 2 summarizes the results of hydraulic testing.


21

The hydraulic conductivities estimated for the MO-2007 wells range from approximately

0.7 feet per day (ft/day) to 120 ft/day, with the majority of estimates between about 10 and

30 ft/day. This magnitude and range of hydraulic conductivities are comparable to previously

reported values for the basin fill (HGC, 2006a and Appendix A). The low end of the range of

estimated hydraulic conductivities were for tests in the deepest monitoring wells (i.e.,

MO-2007-6B, -5C, -4C, -3C, and -1C), which are screened in the lower zone of the basin fill.

Hydraulic conductivity estimates for lower zone monitoring wells ranged from 0.7 ft/day to

11 ft/day. Hydraulic conductivity estimates for wells in the middle zone ranged from 9 ft/day to

31 ft/day. The highest hydraulic conductivity estimate of 118 ft/day was from the test at

MO-2007-2 in the northwest part of the study area where the saturated basin fill is predominantly

sand and sand with gravel. The upper zone of the basin fill is mostly unsaturated.

2.2.4.3 Initial Sampling of MO-2007 Monitor Wells

Initial water quality samples were collected from the MO-2007 wells to document their

water quality. The initial water sampling was conducted after well development and during

aquifer testing conducted at each well. Appendix F describes the methods and provides the

results of the initial water sampling. The results of the initial water quality sampling have been

included in the quarterly groundwater monitoring reports as they became available.

All of the water samples had near-neutral pH, ranging from 7.05 to 7.93. Of the

13 monitoring wells installed for Task 2.4, only wells MO-2007-2, MO-2007-5B, and

MO-2007-5C had sulfate concentrations at or in excess of the action level of 250 mg/L


22

(Figure 4). The water sample from shallow well CW-3 near MO-2007-5B and -5C contained

57.9 mg/L sulfate, indicating vertical zoning in sulfate at that location. Water sampling results

for wells at sites MO-2007-1, -3, -4, and -6 ranged in sulfate concentration from 18.9 mg/L to

136 mg/L. The water quality results for the co-located wells at sites MO-2007-1, -3, -4, and -6

indicate that sulfate concentrations tend to be higher in the lowermost screened interval than in

screened intervals at more shallow depths at the same location. The observation of higher sulfate

in the deeper basin fill, which is generally less permeable than the overlying basin fill, is

perplexing because there does not appear to be a source of sulfate other than the lowermost basin

fill itself. For example, if sulfate in the deepest wells was attributable the sulfate plume, higher

concentrations of sulfate would be expected in the more permeable overlying portion of the basin

fill where sulfate transport would be fastest. A possible explanation for the observed distribution

of sulfate is that the naturally occurring background sulfate concentration is higher in the lower

basin fill, possibly due to the presence of hydrothermal alteration in the underlying bedrock as

observed in MO-2007-2 and MO-2007-3 (Appendix D).

The sulfate concentration data from initial water sampling at the MO-2007 wells better

define the eastern and northern limits of the sulfate plume and provide monitoring facilities

capable of depth-specific sampling in areas between the sulfate plume and drinking water supply

wells. The results of the initial water quality sampling from the newly installed wells will be

verified by subsequent monitoring conducted by the ongoing groundwater monitoring program

pursuant to Task 2.2.


23

Water level measurements at co-located MO-2007 wells indicate slight differences (less

than 4 feet) in water elevation between the upper, middle, and lower well screens at MO-2007-1,

-3, and -4 (Figure 5). Water level differences of approximately 19.7 feet and 16.1 feet are

observed at co-located wells at sites MO-2007-5 and MO-2007-6, respectively. The lowest

water levels at sites MO-2007-5 and MO-2007-6 occur in the lowest screened intervals,

MO-2007-5C and MO-2007-6B. The screen in MO-2007-6B is below a thick clay bed which

may act as a confining layer between the lower screen and the overlying screened interval.

There is no similar low permeability layer above MO-2007-5C, although there are several thin

clayey beds within the screened interval. A possible explanation for the large vertical downward

hydraulic gradients at sites MO-2007-5 and MO-2007-6 may be groundwater pumping at nearby

wells.

2.3 Task 3 - Evaluation of PDSI Groundwater Control System

The interceptor wellfield is a system of 23 wells (the IW-series wells) that pump

sulfate-impacted groundwater at the east edge of the PDSTI (Figures 1 and 4). Groundwater

pumped at the interceptor wellfield is used at the Sierrita Mine. The objective of the interceptor

wellfield is to capture sulfate-impacted seepage at the east edge of the PDSTI before it flows

eastward to the regional basin fill aquifer.

An evaluation of the effectiveness of the interceptor wellfield was included in the Aquifer

Characterization Plan and was conducted to address the requirement of Section III.C.4 of the

Mitigation Order. The evaluation reviewed the development and operation of the PDSTI and the


24

interceptor wellfield, including estimated seepage and sulfate mass capture over time. The

effectiveness of the interceptor wellfield was evaluated based on analysis of water level and

water quality data for the wellfield and the results of numerical simulation of wellfield capture

(M&A, 2007a and 2007b). The evaluation determined that current groundwater pumping

effectively captures sulfate-impacted seepage in the southern portion of the interceptor wellfield,

but not the northern portion from approximately well IW-6A northward (Figures 1 and 4).

Seepage capture at the northern portion of the interceptor wellfield is currently ineffective

because the small saturated thickness of the basin fill aquifer prevents sufficient pumping to

develop an effective hydraulic barrier given the current number of wells. In contrast to the north

half of the interceptor wellfield, the south portion of the wellfield has a greater saturated

thickness that allows the high pumping rates needed to establish effective capture of

sulfate-impacted seepage.

In response to the findings of the interceptor wellfield evaluation, PDSI conducted a

focused feasibility study (FFS) to evaluate potential mitigation alternatives for improving the

effectiveness of the north portion of the interceptor wellfield (HGC, 2007d). The FFS identified

and screened the potential mitigation actions and technologies that could be used to improve the

effectiveness of the northern interceptor wellfield. Mitigation alternatives were developed from

mitigation actions and technologies retained by the screening. The mitigation alternatives

included:

• a new and larger wellfield on PDSI property in the vicinity of the existing northern interceptor wellfield,


25

• new wellfields east of PDSI property where the basin fill saturated thickness is larger,

and

• groundwater recharge via injection wells on PDSI property at the northern interceptor wellfield to enhance seepage recovery.

The mitigation alternatives were evaluated for their implementability, effectiveness, and cost

using the methodology described in the Work Plan.


26


27

3. CONCEPTUAL MODEL FOR THE GROUNDWATER SULFATE PLUME

A preliminary conceptual model describing known and potential sources of sulfate and

the movement of sulfate-bearing groundwater in the vicinity of the PDSTI was presented in the

Work Plan. The conceptual model provides a framework for summarizing information regarding

the source of the sulfate plume and the factors that influence its migration in the environment.

The conceptual model is updated here based on information gathered for the Aquifer

Characterization Plan.

3.1 Sulfate Sources

The primary known source of sulfate is seepage from the PDSTI to the underling basin

fill aquifer. The seepage is due to the gravity drainage of the pore water from the PDSTI. The

pore water consists of water from the tailing slurry delivered to the impoundment, precipitation

that falls on the PDSTI, and surface water discharged to the PDSTI. Seepage from the PDSTI in

2006 was estimated at approximately 7,470 acre feet (M&A, 2007b). Sulfate in the tailing slurry

water results from reagents used in milling, the dissolution of sulfate salts and the oxidation of

sulfide minerals during milling and flotation, and the use of sulfate-bearing water from the

interceptor wellfield in the mill circuit. Sulfate in the reclaim pond results from collection of

tailing slurry water and surface water discharges from the mill.


28

The drainable moisture content of the tailing impoundment represents a finite source of

sulfate-bearing solution that will diminish following the end of mining and mineral processing,

when tailing is no longer deposited and residual moisture drains from the tailing material.

Groundwater in the bedrock upgradient of the tailing impoundment is a second potential

source of sulfate to the basin fill beneath the impoundment. Groundwater samples collected at

piezometers PZ-7 and PZ-8 upgradient of the PDSTI had sulfate concentrations of 360 mg/L and

450 mg/L, respectively, in the third quarter of 2007 (Figure 4 and Appendix B). However, the

contribution of sulfate by bedrock recharge is likely very minor compared to the tailing seepage

because the low permeability of bedrock would limit the sulfate mass flux from the upgradient

area.

Other potential sources of sulfate may occur outside the PDSTI. Studies by the Pima

Association of Governments (PAG) (1983a and 1983b) identified tailing impoundments at other

mines as potential sources. Groundwater sampling results indicate that groundwater in the

vicinity of the Twin Buttes Mine, at the north end of the sulfate plume, contains localized zones

of sulfate in excess of 250 mg/L (Figures 4 and Appendix B).

Another potential source of sulfate is groundwater in the vicinity of the Santa Cruz River.

As documented by Laney (1972) and PAG (1983a), groundwater in the vicinity of the Santa

Cruz River in this part of the Tucson basin can contain greater than 250 mg/L sulfate (Plate 5 in

PAG 1983a). Laney (1972) attributed the sulfate to groundwater derived from gypsiferous

sediment east of the Santa Cruz fault, but irrigation return flow may also add dissolved solids


29

including sulfate. Work conducted for the Aquifer Characterization Plan did not yield additional

information on these potential sources.

Monitoring conducted for Task 2.2 identified a zone of sulfate in excess of 100 mg/L

along the Santa Cruz River channel. The groundwater sampling results indicate that the sulfate

plume from the PDSTI is west of the Santa Cruz River channel and separated from Santa Cruz

River area by a zone of relatively low sulfate (less than 50 mg/L) groundwater (Figure 4 and

Appendix B). At this time, there no apparent interaction between the plume and sulfate-bearing

water along the Santa Cruz River channel.

The results of initial water sampling at sites MO-2007-1, -3, -4, and -6 indicate that

slightly elevated concentrations of sulfate (approximately 75 mg/L to 140 mg/L) are observed in

wells screened in the deeper portions of the basin fill (Figure 4 and Appendix F). The origin of

the sulfate in these deeper wells is uncertain because the locations are outside the area of the

plume and the wells are in sediment with generally lower permeabilities than the overlying basin

fill which has lower sulfate concentrations (Section 2.2.4.3). A possible source of the sulfate in

the deeper basin fill is pyritic and hydrothermally altered bedrock underlying the basin fill,

which may have been incorporated as detritus in the deeper basin fill. It is also possible that the

slightly elevated concentrations of sulfate in these wells are background concentrations for the

deeper basin fill.


30

3.2 Sulfate Migration

Once introduced to the basin fill aquifer, sulfate is transported at the average groundwater

flow velocity because it is a conservative ion and does not attenuate through adsorption or

precipitation at the concentrations and conditions observed in the study area. The direction and

velocity of groundwater flow and sulfate transport are determined by the prevailing hydraulic

gradients and hydraulic properties of the basin fill aquifer.

3.2.1 Hydrostratigraphy of the Basin Fill Aquifer

The Work Plan summarized previous descriptions of the basin fill aquifer in the study

area, including data compiled from wells in the vicinity of the sulfate plume and a

hydrostratigraphic model based on a regional analysis of the Tucson Basin. As noted in the

Work Plan, Davidson (1973) identified three stratigraphic units in the southern Tucson Basin:

Fort Lowell Formation, Tinaja Beds, and Pantano Formation. However, basin fill descriptions in

the Green Valley area typically do not identify these units with the exception of the Pantano

Formation. Geologic data for the MO-2007 wells and previous geologic logging of existing

wells could not be associated confidently with the regional hydrostratigraphic units of

Davidson (1973). Instead, stratigraphic relationships were interpreted based on classification

and comparison of material types intercepted in boreholes.

The hydrostratigraphy and hydraulic properties of the MO-2007-series wells are

discussed in Sections 2.2.4.1 and 2.2.4.2. Appendix G contains geologic cross-sections based on

geologic logs for the MO-2007 wells and borehole data previously reported in the Work Plan.


31

In general terms, the basin fill consists of coarse-grained sediment, primarily sand and

gravel. However, the geologic cross-sections (Appendix G) indicate that in detail there is a

considerable amount of variation in material types with depth and laterally in the basin fill. In

the vicinity of the sulfate plume, a generalized stratigraphic sequence was inferred based on data

from the MO-2007 wells. The stratigraphic sequence identified at the MO-2007 wells

(Section 2.2.4.1) can be generally interpreted through the plume area, although it is not

necessarily identifiable at all locations. Appendix G contains cross-sections showing interpreted

stratagraphic correlations based on the MO-2007 well data and previously reported geologic data

(HGC, 2006a) . The generalized stratigraphy is described below.

The upper zone of basin fill contains sand and gravel with a high proportion of silt and

clay either as discrete layers or as mixtures with the sand and gravel. This zone is between

200 and 600 feet thick in the study area.

Sand and gravel are the predominant material in the middle zone of the basin fill.

Although silt and clay are locally present, they do not form a significant percentage of the middle

zone. The middle zone locally extends to bedrock and elsewhere is underlain by a lower zone of

basin fill.

Geologic logging at the MO-2007 wells identified a lower zone of basin fill that varied

from the overlying middle zone by containing one or more of the following: greater amounts of

silt and clay, the lack of gravel, zones of moderate induration, and increased calcium carbonate

based on reaction with hydrochloric acid. Correlation of the lower zone of basin fill from well to


32

well in the vicinity of the plume is difficult on the basis of pre-existing well logs, but some

general observations can be made. Where identified in the MO-2007 wells, the elevation of the

top of the lower zone appears to correlate laterally either in elevation or stratigraphic position

with material previously identified as Pantano Formation (e.g., between MH-13 and MO-2007-5

on Cross-Section F-F’, at ESP-1 through ESP-4 between MO-2007-5 and MO-2007-1 on

Cross-Section C-C’, see Appendix G). Although they share the apparent correlation of depth and

position beneath the middle zone, there are distinct differences between projected lower unit

materials. For example, the lower zone identified as weakly to moderately lithified Pantano

Formation at MH-13 is a gravel, whereas the lower zone at MO-2007-5 and 4 is unconsolidated

to moderately indurated silty sand to sand with silt. Projecting the lower zone north from

MO-2007-5, the lower zone is described as a variably indurated sand at MO-2007-4, poorly

consolidated to cemented, variably calcareous conglomerate and quartzose sandstone of the

Helmet Peak Fanglomerate (equivalent to the Pantano Formation) at ESP-1, -2, -3, and -4,

uniform sand at MO-2007-3, and silty sand at MO-2007-1. There is insufficient information to

extrapolate the lower basin fill unit east of the plume. A lower unit of basin fill can be projected

west of MH-13 to wells IW-12, IW-14, and IW-15, where Pantano Formation is identified

beneath sand and gravel at the interceptor wellfield (Cross-Section A-A’, Appendix G).

The geologic data for wells in the vicinity of the plume were interpreted in terms of an

informal hydrostratigraphy consisting of an upper zone of sand-containing zones or layers of silt

and clay, a middle zone of sand and gravel, and a lower zone of unconsolidated to moderately

consolidated sand to silty and clayey sand to gravel. Based on this interpretation, the upper


33

stratigraphic zone of basin fill is largely unsaturated and the basin fill aquifer is comprised

primarily of the saturated middle and lower stratigraphic zones.

The stratigraphic interpretation provided here is provisional because of the inherent

uncertainties in projecting units based on the available data. First, stratigraphic interpretation is

difficult in sequences of coarse-grained fluvial sediments such as the basin fill because the

differences between units can be subtle and gradational, based on relatively minor variations in

the percentages of different materials. Second, the processes that deposit coarse fluvial sediment

can result in lateral and vertical facies changes that limit the extent and complicate the pattern of

occurrence of different stratigraphic units. Third, the use of previously reported geologic data

for drill cuttings sampled by different techniques and logged by numerous individuals for

different purposes over time, complicates stratigraphic interpretation because the data are not

always comparable.

Hydraulic conductivity estimates for wells in the vicinity of the plume are plotted on

cross-sections in Appendix H based on data previously presented in the Work Plan and verified

for Task 2.1 (Appendix A). In general, most of the estimated hydraulic conductivities shown on

the cross-sections range between 5 and 50 ft/day. With the exception of wells in the lowermost

basin fill, hydraulic conductivity estimates for wells screened both over the entire saturated

thickness of the basin fill and wells with shorter screened intervals range about an order of

magnitude and are generally about several tens of feet per day. The cross-sections show that

there is a tendency for lower hydraulic conductivities in the lower unit of basin fill, although

there is not always a significant difference between the hydraulic conductivities of the middle


34

and lower zones of the basin fill (e.g., sites MO-2007-3 and MO-2007-4). There is also a general

tendency for hydraulic conductivity to increase slightly from south to north, with the highest

hydraulic conductivities in the northern portion of the plume (e.g., CW-7 and MO-2007-2).

As discussed in the Work Plan, the bedrock is significantly less permeable than the

overlying basin fill aquifer based on the results of hydraulic testing of bedrock at MH-25 within

the plume and elsewhere in the vicinity of the PDSTI. For this reason, the bedrock aquifer is not

considered to have significant groundwater flow or the potential to transport sulfate relative to

the basin fill aquifer.

The information from pre-existing boreholes and drilling conducted for this study is

consistent with a hydrostratigraphic model of a continuous, unconfined basin fill aquifer

consisting of well-sorted, coarse-grained sediment possessing a generalized three-layer

stratigraphy. Although there are variations in the hydraulic conductivity of the basin fill aquifer

indicating the middle zone has a higher permeability than the deep zone (e.g., low permeability

in the deep screened intervals at MH-13 and the MO-2007 wells and high inflows suggesting

high permeability in the middle zone at ESP-2 and ESP-4), large-scale features with significant

(two orders of magnitude or more) hydraulic conductivity contrasts have not been identified in

the basin fill aquifer. Thus, there is no evidence of regionally extensive heterogeneities that can

cause preferential flow paths, such as laterally extensive aquitards or high permeability units,

although variations in the velocity of groundwater flow and sulfate transport may occur due to

local scale differences in hydraulic properties.


35

3.2.2 Sulfate Distribution in the Basin Fill Aquifer

The lateral distribution of sulfate in the basin fill aquifer is shown on Figure 4. The

extent of the sulfate plume as defined by the 250 mg/L contour is shown on Figure 1. Within the

plume, elevated sulfate occurs throughout the thickness of the saturated basin fill aquifer with the

exception of the uppermost portions of the basin fill aquifer at MH-25A, MH-26A, and in the

lower most part of the aquifer at MH-13C (Figure 4 and Appendix B). On the eastern margin of

the plume, elevated sulfate is observed in an apparent permeable zone at a depth of

approximately 650 and 800 feet in well ESP-4 (Section 2.2.3) and at wells MO-2007-5B

(screened from 660 to 960 ft bgs) and MO-2007-5C (screened from 1,150 to 1,350 ft bgs)

(Section 2.2.4.3). The lateral and vertical distributions of sulfate on the margins of the plume

can be influenced by local-scale aquifer heterogeneities and hydraulic conditions.

3.2.3 Sulfate Transport

Sulfate-bearing seepage from the tailing impoundment infiltrates into the underlying

basin fill, mixes with groundwater recharge by surface infiltration and groundwater inflow from

the upgradient bedrock, and flows eastward. Sulfate-bearing seepage is intercepted through

groundwater pumping within the interceptor wellfield. Current pumping at the southern portion

of the interceptor wellfield effectively captures most of the sulfate-bearing seepage in the area.

Capture of sulfate-bearing seepage by the northern portion of the interceptor wellfield is

currently incomplete (M&A, 2007), an issue that is being addressed by the FFS (HGC, 2007d).

Impacted groundwater that is not intercepted at the wellfield or that has already moved

downgradient of the interceptor wellfield flows north-northeasterly as it enters the northerly


36

flowing regional groundwater system in the basin fill aquifer near Green Valley (Section 2.2.2.3,

Figure 5). The sharp north-south orientation of the eastern plume boundary (Figure 4) is due to

the northward groundwater flow that occurs as the hydraulic gradient becomes northerly and

groundwater from the PDSTI area mixes with regional groundwater flow in the central part of

the basin.

In addition to regional hydrologic conditions, groundwater flow and sulfate transport can

be influenced by local sites of groundwater pumping and recharge. For example, pumping at a

well in the immediate vicinity of the plume margin can induce hydraulic gradients that cause the

plume to migrate toward the well. This can be seen in sulfate concentration data for well ESP-1,

which increased when the well was pumped for several months and decreased when pumping

ceased (see data for ESP-1 Table B.2 in Appendix B, pumping at ESP-1 was reduced in the

second quarter of 2007 and stopped in the third quarter 2007). Collectively, groundwater

pumping at wells outside of the plume and recharge along the Santa Cruz River due to stream

channel infiltration or groundwater recharge projects can influence the migration and location of

the sulfate plume, but the degree of influence will depend on the location, magnitude, and

duration of pumping or recharge.


37

4. NUMERICAL MODEL OF GROUNDWATER FLOW AND TRANSPORT

A numerical model of groundwater flow and sulfate transport was developed for Task 4

of the Aquifer Characterization Plan. The model and the results of its calibration to current

conditions are generally discussed in this section. Appendix I reports the scope, construction,

calibration, and sensitivity of the model.

The model is regional in scale, but was developed to focus on the area of the sulfate

plume for the purpose of evaluating mitigation alternatives. The model was calibrated to past

and present measured hydraulic heads and sulfate concentrations to provide a reasonable

approximation of the groundwater flow system and the sulfate plume. The model is intended for

use as a predictive tool to simulate the fate and transport of sulfate in the vicinity of the PDSTI

under various potential mitigation actions such as groundwater pumping. Groundwater flow and

transport simulations with the model will be used to provide conceptual design bases for

potential mitigation actions considered in the Feasibility Study. Any use of the model outside

the area of the sulfate plume may require additional aquifer characterization and model

refinement.

The model is calibrated on simulation of conditions from 1940 through 2006 including

groundwater pumping at wells in the vicinity of the PDSTI and seepage from the PDSTI. The

results of the calibrated model are presented as contour maps comparing measured and simulated

water level elevations and sulfate concentrations. Figures 6 and 7 show the water elevation and

sulfate concentration predictions of the calibrated model. Appendix I contains illustrations of the


38

transient simulation of time series water level and sulfate concentration data. In viewing and

interpreting the results of the calibrated model, it is important to understand that a regional model

of groundwater flow and sulfate transport for a system such as the sulfate plume from the PDSTI

cannot match every detail of the observed system. The model is necessarily a generalized

representation of the aquifer because data on the spatial distribution of material types and

hydraulic properties are incomplete, as are data on the temporal and spatial distribution of

sources and sinks. Therefore, the objective of calibration was to match groundwater elevations

and sulfate concentrations in the vicinity of the plume, and the overall temporal development of

the plume. Because the majority of available hydrologic data are for the immediate vicinity of

the plume, prediction in areas far from the plume may have greater uncertainty because of the

lack of site-specific information for those areas.

Comparison of Figures 5 and 6 shows the calibrated model matches the current water

level elevations and potentiometric configuration fairly well. The measured and predicted

distribution of sulfate is also good based on comparison of Figures 4 and 7. The model is

considered suitable for the purpose of simulating plume behavior under potential mitigation

actions, although the model does have limitations in areas outside of the plume.

As discussed in the Work Plan, the calibrated model will be used in the Feasibility Study

to develop and evaluate potential plume mitigation actions. Simulations of potential mitigation

actions will be conducted to predict future conditions of hydraulic head and sulfate distribution

over time as a basis for evaluating effectiveness. Simulations of future conditions for the


39

Feasibility Study will consider future pumping and water supply development described by water

system plans.


40


41

5. CONCLUSIONS

Hydrologic investigations required by the Aquifer Characterization Plan were

implemented and completed between November 2006 and December 2007 in accordance with

the Work Plan schedule approved by ADEQ. This Aquifer Characterization Report is the final

task required for the Aquifer Characterization Plan, although groundwater monitoring for Task

2.2 will be ongoing until it is superseded by the recommendations of the Mitigation Plan.

Additional requirements of the Work Plan that are currently in development are the Feasibility

Study due April 30, 2008 and the Mitigation Plan due June 30, 2008.

The results of the Aquifer Characterization Plan have accomplished the objectives stated

in the Work Plan (Section 1.2). Specific accomplishments include:

• A well inventory of drinking water supply wells within one-mile downgradient and crossgradient of the sulfate plume was completed, including sampling drinking water supplies to identify any impacted wells. No active drinking water supplies were found to be impacted by sulfate from the PDSTI. An interim action plan was put in place to identify and implement mitigation actions in the event a drinking water supply becomes impacted. Prior to entering the Mitigation Order, PDSI replaced two CWC drinking water supply wells.

• The vertical and lateral extent of the sulfate plume was determined by the installation and water quality sampling of 13 new monitoring wells at six locations along the margin of the plume, rehabilitation and sampling of three previously inactive wells (TMM-1, NP-2, and CW-3) on the margin of the plume, completion of four quarters of plume monitoring and two regional monitoring events to determine the extent of the plume and characterize regional water quality, and depth-specific sampling at four wells to evaluate vertical trends. PDSI collected and analyzed more than 350 water quality samples to evaluate the extent of sulfate.

• The effectiveness of the interceptor wellfield was evaluated based on a thorough

review of its construction and operational history, including estimation of seepage and sulfate mass capture. When the northern portion of the interceptor wellfield was found to be ineffective, PDSI initiated the FFS to identify a more effective control strategy.


42

• A numerical model of groundwater flow and sulfate transport was developed to

evaluate the fate and transport of the sulfate plume. The model is conditioned on site-specific information for the basin fill aquifer and the results of calibration to a 66-year record of groundwater pumping and a 47-year record of tailing emplacement. The model will be used in the Feasibility Study to evaluate the effectiveness of mitigation actions on control of the sulfate plume.


43

6. REFERENCES Arizona Department of Environmental Quality (ADEQ). 2006. Correspondence from Cynthia

Campbell, ADEQ, to John Brack, Phelps Dodge Miami, Inc., Re: Mitigation Order on Consent Docket No. P-50-06 - Work Plan. November 15, 2006.

Davidson, E.S. 1973. Geohydrology and Water Resources of the Tucson Basin, Arizona, USGS

Water-Supply Paper 1939-E, 81p. Errol L. Montgomery & Associates (M&A). 2007a. Evaluation of the Current Effectiveness of

the Interceptor Wellfield Phelps Dodge Sierrita Mine, Pima County, Arizona. February 26, 2007

M&A. 2007b. Revised Report: Evaluation of the Current Effectiveness of the Sierrita

Interceptor Wellfield, Phelps Dodge Sierrita Mine, Pima County, Arizona. November 14, 2007.

Errol L. Montgomery & Associates and Dames & Moore (M&A and Dames & Moore). 1994.

Aquifer Protection Permit Application, Sierrita Operation, Cyprus Sierrita Corporation, Pima County, Arizona. Volumes I and II. September 7, 1994.

Hydro Geo Chem, Inc. (HGC). 2006a. Work Plan to Characterize and Mitigate Sulfate with

Respect to Drinking Water Supplies in the Vicinity of the Phelps Dodge Sierrita Tailing Impoundment, Pima County, Arizona. August 11, 2006, revised October 31, 2006.

HGC. 2006b. Well Inventory Report for Task 1 of Aquifer Characterization Plan for Mitigation

Order on Consent No. P-50-06. December 20, 2006. HGC. 2006c. Interim Action Identification Technical Memorandum, Mitigation Order on

Consent Docket No. P-50-06, Pima County, Arizona. December 22, 2006. HGC. 2006d. Groundwater Monitoring Report, Fourth Quarter 2006, Tasks 2.2 and 2.3 of

Aquifer Characterization Plan, Mitigation Order on Consent Docket No. P-50-06. December 29, 2006.

HGC. 2007a. First Quarter 2007, Groundwater Monitoring Report, Tasks 2.2 and 2.3 of Aquifer

Characterization Plan, Mitigation Order on Consent Docket No. P-50-06. March 30, 2007.

HGC. 2007b. Second Quarter 2007, Groundwater Monitoring Report, Tasks 2.2 and 2.3 of

Aquifer Characterization Plan, Mitigation Order on Consent Docket No. P-50-06. June 28, 2007.


44

HGC. 2007c. Third Quarter 2007, Groundwater Monitoring Report, Tasks 2.2, 2.3, and 2.4 of Aquifer Characterization Plan, Mitigation Order on Consent Docket No. P-50-06. June 28, 2007.

HGC. 2007d. Focused Feasibility Study for the Northern Portion of the Interceptor Wellfield,

Phelps Dodge Sierrita Mine Tailing Impoundment, Mitigation Order on Consent Docket No. P-50-06. December 28, 2007.

Laney, R.L. 1972. Chemical Quality of Water in the Tucson Basin, Arizona. U.S.G.S.

Geological Survey Water Supply Paper 1939-D. Pima Association of Governments (PAG). 1983a. Region Wide Groundwater Quality in the

Upper Santa Cruz Basin Mines Task Force Area. September 1983. PAG. 1983b. Ground-Water Monitoring in the Tucson Copper Mining District. September

1983.


45

7. LIMITATIONS STATEMENT

The opinions and recommendations presented in this report are based upon the scope of

services and information obtained through the performance of the services, as agreed upon by

HGC and the party for whom this report was originally prepared. Results of any investigations,

tests, or findings presented in this report apply solely to conditions existing at the time HGC’s

investigative work was performed and are inherently based on and limited to the available data

and the extent of the investigation activities. No representation, warranty, or guarantee, express

or implied, is intended or given. HGC makes no representation as to the accuracy or

completeness of any information provided by other parties not under contract to HGC to the

extent that HGC relied upon that information. This report is expressly for the sole and exclusive

use of the party for whom this report was originally prepared and for the particular purpose that

it was intended. Reuse of this report, or any portion thereof, for other than its intended purpose,

or if modified, or if used by third parties, shall be at the sole risk of the user.


46

TABLES

TABLE 1Summary of MO-2007-Series Wells

WELL NAMEADWR WELL REGISTRY NUMBER

UTM NORTHING (NAD 83, meters)

UTM EASTING (NAD 83, meters)

DRILLED DEPTH (ft bls)

CASING DEPTH (feet)

CASING DIAMETER

(inch)

DEPTH TO TOP OF SCREEN

(ft bls)

DEPTH TO BOTTOM OF SCREEN

(ft bls)

SCREEN LENGTH

(feet)

MEASURING POINT ELEVATION

(NAVD 88, ft amsl)

DATE MEASURED

DEPTH TO WATER BELOW MEASURING

POINT (feet)

STATIC WATER LEVEL

ELEVATION (ft amsl)

MO-2007-1A 907342 3529331.380 500016.947 620 610 5 460 600 140 2967.15 07/30/07 425.87 2541.28

MO-2007-1B 907210 3529325.119 500021.574 920 910 5 740 900 160 2966.35 07/30/07 425.67 2540.68

MO-2007-1C 907209 3529328.959 500013.405 1260 1190 5 1020 1180 160 2964.34 07/30/07 423.87 2540.47

MO-2007-2 906765 3527621.102 497912.410 740 685 5 520 680 160 3153.61 08/09/07 575.30 2578.31

MO-2007-3B 906816 3528508.801 500522.491 960 950 5 740 940 200 2910.75 09/10/07 359.38 2551.37

MO-2007-3C 906817 3528508.743 500529.713 1430 1330 5 1160 1320 160 2910.09 07/05/07 356.30 2553.79

MO-2007-4A 907213 3525634.956 500383.682 580 570 5 360 560 200 2923.47 10/09/07 307.67 2615.80

MO-2007-4B 907212 3525613.952 500380.947 960 950 5 700 940 240 2923.22 10/11/07 308.72 2614.50

MO-2007-4C 907211 3525624.484 500382.217 1153 1140 5 1090 1130 40 2923.49 08/12/07 307.13 2616.36

MO-2007-5B 907456 3523743.376 500013.850 980 970 5 660 960 300 2943.42 10/12/07 268.27 2675.15

MO-2007-5C 907457 3523736.459 500014.152 1370 1360 5 1150 1350 200 2944.33 08/23/07 294.04 2650.29

310 390 80

430 610 180

MO-2007-6B 907606 3521849.495 498367.887 1060 950 5 780 940 160 3041.95 10/04/07 319.17 2722.78

CW-3 627483 3523809.985 500047.663 501 500 16 182 500 318 2941.44 06/06/07 265.35 2676.09

NP-2 605898 3528517.116 500582.904 515 515 12 331 515 1 1841 2907.05 06/04/07 351.50 2555.55

Notes:ADWR = Arizona Department of Water ResourcesUTM = Universal Transverse Mercator (Zone 12) NAD 83, meters = North American Datum of 1983NAVD 88 = North American Vertical Datum of 1988ft amsl = feet above mean sea levelft bls = feet below land surface

1 depth to bottom of screen and screen length are not provided in the ADWR well registry and therefore estimated

Existing Wells at MO-2007 Sites

3042.49 2738.8910/02/07 303.60620 5MO-2007-6A 907607 3521842.050 498367.161 630

H:\78300\78306.4\Drilling Results\MO-2007-Well List.xls: Table 1

TABLE 2Summary of Hydraulic Parameters for MO-2007-Series Wells

Well T (ft2/day) S b (ft) Kh (ft/day)

MO-2007-1A 20,000 0.001 815 25

MO-2007-1B 25,000 0.001 815 31

MO-2007-1C 7,000 0.001 815 8.6

MO-2007-2 13,000 0.001 110 118

MO-2007-3B 17,700 0.001 1,060 17

MO-2007-3C 10,100 - 11,600 0.00016 - 0.001 1,060 9.5 - 11

MO-2007-4A 7,500 0.005 835 9

MO-2007-4B 10,000 - 20,000 0.005 - 0.1 835 12 - 24

MO-2007-4C 8,680 - 9,000 0.001 835 10-11

MO-2007-5B 31,200 0.001- 0.1 1085 29

MO-2007-5C 785 0.001 1085 0.72

MO-2007-6A 4,150 - 17,000 0.0057 325 - 655 12 - 31

MO-2007-6B 210-750 0.001 190 - 655 1.1

Notes:

T = Transmissivity

S = Storage coefficientb = Assumed aquifer thicknessKh = horizontal hydraulic conductivity calculated as T/bft/day = feet per day

ft 2 /day = feet squared per day

H:\78300\REPORTS\AC Report\ACR Main\Table 2 MO Wells Hydraulic Parameters Summary-1.xls: Table 2

TABLE 3Water Quality Data for Initial Sampling of

MO-2007-Series Wells

Well NameADWR 55 Well

Registry Number

Sample DateField pH

(SU)Field EC(µS/cm)

Field Temp(deg C)

Sulfate, total

Sulfate, dissolved

Chloride, dissolved

Fluoride, dissolved

Nitrate as N, dissolved

Nitrite as N, dissolved

Nitrate/Nitriteas N, dissolved

Calcium, dissolved

Magnesium, dissolved

MO-2007-1A 907342 08/08/07 7.17 370 29.0 19.2 19.2 8.4 0.4 0.54 < 0.01 0.54 40.4 6.4

MO-2007-1B 907210 08/02/07 7.41 321 30.7 18.9 18.9 12.4 0.6 0.71 < 0.01 0.71 32.4 4.3

MO-2007-1C 907209 07/31/07 7.35 523 27.9 114 112 22.4 0.5 0.82 < 0.01 0.82 57.5 9.3

MO-2007-2 906765 06/14/07 7.05 1372 32.2 596 591 28.3 0.3 0.94 < 0.01 0.94 196.0 35.5

NP-2 1 605898 06/04/07 7.20 411 25.9 41.3 41.2 9.1 0.2 0.34 < 0.01 0.34 50.3 10.9

MO-2007-3B 906816 09/10/07 7.53 373 28.7 38 38 7.0 0.5 0.33 < 0.01 0.33 31.5 2.8

MO-2007-3C 906817 06/28/07 7.93 570 32.2 136 136 11.4 3.1 0.30 < 0.01 0.30 28.2 1.4

MO-2007-4A 907213 10/09/07 7.46 412 27.5 37.2 37 10.2 0.3 0.93 < 0.01 0.93 42.8 6.2

MO-2007-4B 907212 10/11/07 7.93 376 26.4 37.5 37.6 9.1 0.6 0.77 < 0.01 0.77 41.6 4.3

MO-2007-4C 907211 08/16/07 7.62 472 35.2 78.6 78.7 11.8 5.0 0.48 < 0.01 0.48 13.0 0.3

CW-3 1 627483 06/06/07 7.74 449 25.3 58.7 57.9 17.7 0.3 2.92 < 0.01 2.92 56.1 10.9

MO-2007-5B 907456 10/12/07 7.63 1150 29.9 392 402 44.5 1.2 1.97 0.01 1.98 84.8 3.7

MO-2007-5C 907457 08/23/07 7.46 780 31.4 252 248 12.0 2.1 0.13 0.02 0.15 30.0 1.4

MO-2007-6A 907607 10/02/07 7.52 405 28.5 27 26.5 10.5 0.3 0.99 < 0.01 0.99 36.3 5.4

MO-2007-6A [DUP] 907607 10/02/07 7.52 405 28.5 26.5 26.5 10.5 0.3 0.98 < 0.01 0.98 36.4 5.4

MO-2007-6B 907606 10/04/07 7.70 483 33.1 93.5 93.6 10.9 0.5 0.67 0.02 0.69 28.1 2.9

Notes:

ADWR = Arizona Department of Water ResourcesSU = Standard UnitsµS/cm = microsiemens per centimeterdeg C = degrees CelsiusTDS = Total Dissolved Solidsmeq/L = milliequivalent per literDUP = Duplicate Sample

All units are in milligrams per liter (mg/L) unless otherwise noted

1 = Existing well designated as monitoring well for sampling the shallow zone of the basin fill aquifer

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TABLE 3Water Quality Data for Initial Sampling of

MO-2007-Series Wells

Well NameADWR 55 Well

Registry Number

Sample Date

MO-2007-1A 907342 08/08/07

MO-2007-1B 907210 08/02/07

MO-2007-1C 907209 07/31/07

MO-2007-2 906765 06/14/07

NP-2 1 605898 06/04/07

MO-2007-3B 906816 09/10/07

MO-2007-3C 906817 06/28/07

MO-2007-4A 907213 10/09/07

MO-2007-4B 907212 10/11/07

MO-2007-4C 907211 08/16/07

CW-3 1 627483 06/06/07

MO-2007-5B 907456 10/12/07

MO-2007-5C 907457 08/23/07

MO-2007-6A 907607 10/02/07

MO-2007-6A [DUP] 907607 10/02/07

MO-2007-6B 907606 10/04/07

Notes:

ADWR = Arizona Department of Water ResourcesSU = Standard UnitsµS/cm = microsiemens per centimeterdeg C = degrees CelsiusTDS = Total Dissolved Solidsmeq/L = milliequivalent per literDUP = Duplicate Sample

All units are in milligrams per liter (mg/L) unless otherwise noted

1 = Existing well designated as monitoring well for sampling the shallow zone of the basin fill aquifer

Potassium, dissolved

Sodium, dissolved

Total Alkalinity

Bicarbonate as CaCO3

Carbonate as CaCO3

Hydroxide as CaCO3

Residue, Filterable (TDS) @

180ºC

TDS (calculated)

TDS Ratio (measured/ calculated)

Sum of Anions (meq/L)

Sum of Cations (meq/L)

Cation-Anion Balance (%)

3.0 30.4 164 164 < 2 < 2 250 209 1.20 3.9 3.9 0.0

3.2 40.5 140 140 < 2 < 2 220 199 1.11 3.6 3.8 2.7

4.8 49.3 124 124 < 2 < 2 380 334 1.14 5.5 5.9 3.5

7.7 73.5 108 108 < 2 < 2 1060 1000 1.06 15.4 16.1 2.2

3.9 31.7 169 169 < 2 < 2 280 250 1.12 4.5 4.9 4.3

3.1 44.1 134 134 < 2 < 2 250 209 1.20 3.7 3.8 1.3

3.3 93.4 103 103 < 2 < 2 380 340 1.12 5.4 5.7 2.7

3.3 37.1 160 155 5 < 2 270 239 1.13 4.3 4.3 0.0

2.9 35.7 143 143 < 2 < 2 230 221 1.04 3.9 4.0 1.3

1.9 80.8 103 101 2 < 2 310 256 1.21 4.3 4.2 -1.2

3.0 30.5 140 140 < 2 < 2 300 273 1.10 4.7 5.1 4.1

5.5 164.0 95 95 < 2 < 2 780 771 1.01 11.8 11.9 0.4

7.1 129.0 71 71 < 2 < 2 540 473 1.14 7.0 7.4 2.8

3.8 39.8 164 164 < 2 < 2 920 225 4.09 4.2 4.1 -1.2

3.8 40.0 163 163 < 2 < 2 260 225 1.16 4.2 4.1 -1.2

11.3 60.6 125 119 5 < 2 400 287 1.39 4.8 4.6 -2.1

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Documents

AQUIFER CHARACTERIZATION REPORT TASK 5 OF AQUIFER … · 2007. 12. 28. · • Task 4 – Sulfate Fate and Transport Evaluation • Task 5 – Preparation of the Aquifer Characterization